The specific objective of this work is to define contraction, relaxation, and control of actomyosin get in terms of the chemical reactions, between ATP, magnesium, calcium, and protein, which appear to occur at a number of different specialized contractile and control sites - only one of thesebeing the enzymatic site that catalyzes ATP hydrolysis. Despite the complexity of having to deal with more than one kind of site, it has been possible to distinguish them by experimental analysis, and future work is planned to further define their chemistry and function. The approach to this analytical problem includes the use of chemicals and physical conditions found to be selectively modify one or another protein site, together with kinetic analyses that compare and correlate hydrolysis, 180-exhange, superprecipitation and clearing of natural actomyosin. In this work actomyosin is used as a model of muscle and the more general objective of these studies is to contribute to a more precise biochemical understanding of muscular contraction and its control.